CLC (Charcot-Leyden Crystal) protein, or eosinophil leukocyte lysophospholipase, is a typical protein of human eosinophil leukocytes and basophiles. Recently it has been shown that CLC protein is structurally and functionally related to the galectin family of beta-galactoside binding proteins. The galectin family of proteins share several common characteristics, such as a linear ligand-binding site (coded by an exon). In addition, the genes encoding the CLC protein gene is structurally similar to genes encoding galectin.
Galectins are in turn a member of the animal beta-galactoside-binding lectin family. In mammals lectins are classified into four types: type C, type P, pentraxins and galectins. Galectins are sulfhydryl dependent, and specifically bind to beta-galactoside). Galectins can be further divided into two types, type S and type S-Lac. Galectins are defined by two essential biochemical characteristics: 1) a unique and conserved amino acid sequence, and 2) affinity with beta-galactose. The coding sequence of a CLC protein gene has four exons, including a highly conserved beta-galactoside binding site encoded by exon III, i.e., a carbohydrate binding domain. CLC protein is a single peptide and is secreted to the cytoplasm after synthesis. It is a soluble protein, secreted through non-typical pathway. All above-mentioned are typical characteristics of galectin. Therefore, we consider the novel protein of the invention a member of galectin family, and name it as human galectin 15.